The sharp rise in the price of crude oil in recent years has resulted in oil or gas wells being dug deeper and in severely corrosive environments. Digging deeper oil or gas wells in harsh environments requires using oil well pipes that have high strength and possess excellent corrosion resistance and stress corrosion cracking resistance.
Oil and natural gas contain corrosive substances such as carbon dioxide gas, hydrogen sulfide, and chlorine ions. Material used for drilling oil or gas wells must therefore possess excellent resistance to such corrosive substances. Stress corrosion cracking is a main corrosive factor particularly in environments at temperatures of 150° C. or higher and containing hydrogen sulfide at 1 atm or higher. High stress corrosion cracking resistance is therefore required in materials used in environments containing hydrogen sulfide.
Oil and gas wells are being drilled deeper in recent years due to the increasing demand for oil and natural gas. The materials used in these deeper oil and gas wells must possess higher strength and also provide corrosion resistance against carbon dioxide gas, hydrogen sulfide, and chlorine ions. Examples of materials possessing excellent corrosion resistance in corrosive environments are the Cr—Ni alloy materials disclosed in patent document 1, patent document 2, and patent document 3. These patent documents 1 to 3 also reveal that increasing the N content is effective in increasing the strength of Cr—Ni alloy material. A drawback, however, is that the high strength alloy reinforced in this manner might have poor hot reduction rate. In view of this problem, elements such as Ca, Mg, and Si, and REM (rare earth metal) are added to improve the hot reduction rate.
The Cr—Ni alloy material disclosed in patent document 4 reveals that reducing the Mo content improves the hot reduction rate. A problem, however, is that cold working with high reduction rate is required in the case where the N content is low and higher strength is needed, involving deterioration of ductility and toughness.
Patent document 5 discloses, as a material having excellent corrosion resistance in an acidic environment or a seawater environment as well as having excellent hot workability, a super-austenitic stainless steel with a higher Mn and Mo content, and containing Ce, Ca, or a similar element. The material disclosed in patent document 5, however, is inadequate when higher hot workability is needed. This material further has such a problem that ductility and toughness might deteriorate if the material is subjected to cold working at high reduction rate to increase the material strength.
[Patent Document 1] Japanese Unexamined Patent Publication No. S57-203735
[Patent Document 2] Japanese Unexamined Patent Publication No. S57-207149
[Patent Document 3] Japanese Unexamined Patent Publication No. S58-210155
[Patent Document 4] Japanese Unexamined Patent Publication No. H11-302801
[Patent Document 5] Japanese Unexamined Patent Publication No. 2005-509751